In multiple-input multiple-output (MIMO) systems (including massive MIMO systems), it is desirable to have coherently driven antennas with closely controlled phases and delays between antennas. Similarly, coherency among individual antennas of an antenna array is desirable for beamforming and beam steering. Generally, coherency means that in a system of multiple antenna branches, there is a defined phase-coherent relationship between the branches. Any loss of coherence (e.g., due to noise and/or distortion) erodes the system's ability to separate signal modes (e.g., in a MIMO system) or form beams (e.g., in an antenna array) because antenna signals become smeared over space and time.
A challenge in achieving a coherent system is the complexity of generating and distributing the local oscillator (LO) signals that are used for frequency conversion in the antenna branches, particularly in microwave and high frequency ranges. A conventional approach for distributing an LO signal among multiple antenna branches is to use metallic transmission lines to distribute the LO signals. However, such transmission lines exhibit significant loss at microwave and higher frequencies, thus needing more signal amplification and resulting in higher noise, greater distortion, higher power consumption and increased costs. Other conventional approaches may use lower-loss waveguides or optical fibers, however such approaches are expensive to manufacture and have difficulty in achieving phase and delay matching at shorter wavelengths.
It would be useful to provide a system for distributing LO signals over multiple antenna branches, which addresses the problems of excessive losses and/or distortion, particularly for higher frequencies.